Add Hash#map_values to ActiveSupport

Didn't get a chance to convert existing code, I'll skim through the code
base to make use of this later this afternoon.

activesupport/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Add `Hash#transform_values` to simplify a common pattern where the values of a
+    hash must change, but the keys are left the same.
+
+    *Sean Griffin*
+
 *   Always instrument `ActiveSupport::Cache`.
 
     Since `ActiveSupport::Notifications` only instrument items when there

activesupport/lib/active_support/core_ext/hash.rb

@@ -6,3 +6,4 @@
 require 'active_support/core_ext/hash/keys'
 require 'active_support/core_ext/hash/reverse_merge'
 require 'active_support/core_ext/hash/slice'
+require 'active_support/core_ext/hash/transform_values'

activesupport/lib/active_support/core_ext/hash/transform_values.rb

@@ -0,0 +1,21 @@
+class Hash
+  # Returns a new hash with the results of running +block+ once for every value.
+  # The keys are unchanged.
+  #
+  #   { a: 1, b: 2, c: 3 }.transform_values { |x| x * 2 }
+  #   # => { a: 2, b: 4, c: 6 }
+  def transform_values(&block)
+    result = self.class.new
+    each do |key, value|
+      result[key] = yield(value)
+    end
+    result
+  end
+
+  # Destructive +transform_values+
+  def transform_values!
+    each do |key, value|
+      self[key] = yield(value)
+    end
+  end
+end

activesupport/test/core_ext/hash/transform_values_test.rb

@@ -0,0 +1,49 @@
+require 'abstract_unit'
+require 'active_support/core_ext/hash/indifferent_access'
+require 'active_support/core_ext/hash/transform_values'
+
+class TransformValuesTest < ActiveSupport::TestCase
+  test "transform_values returns a new hash with the values computed from the block" do
+    original = { a: 'a', b: 'b' }
+    mapped = original.transform_values { |v| v + '!' }
+
+    assert_equal({ a: 'a', b: 'b' }, original)
+    assert_equal({ a: 'a!', b: 'b!' }, mapped)
+  end
+
+  test "transform_values! modifies the values of the original" do
+    original = { a: 'a', b: 'b' }
+    mapped = original.transform_values! { |v| v + '!' }
+
+    assert_equal({ a: 'a!', b: 'b!' }, original)
+    assert_same original, mapped
+  end
+
+  test "indifferent access is still indifferent after mapping values" do
+    original = { a: 'a', b: 'b' }.with_indifferent_access
+    mapped = original.transform_values { |v| v + '!' }
+
+    assert_equal 'a!', mapped[:a]
+    assert_equal 'a!', mapped['a']
+  end
+
+  # This is to be consistent with the behavior of Ruby's built in methods
+  # (e.g. #select, #reject) as of 2.2
+  test "default values do not persist during mapping" do
+    original = Hash.new('foo')
+    original[:a] = 'a'
+    mapped = original.transform_values { |v| v + '!' }
+
+    assert_equal 'a!', mapped[:a]
+    assert_nil mapped[:b]
+  end
+
+  test "default procs do not persist after mapping" do
+    original = Hash.new { 'foo' }
+    original[:a] = 'a'
+    mapped = original.transform_values { |v| v + '!' }
+
+    assert_equal 'a!', mapped[:a]
+    assert_nil mapped[:b]
+  end
+end